Weed Control in Pumpkins and Winter Squash

First, know your weeds.

This is beyond knowing you have general issues with grasses or broadleaves. Some cultural techniques and herbicide chemistries are more effective against some weeds than others. Being familiar with the specific weed issues in the fields you are planting into will help you tailor your weed control program for success. The row crops side of the University of Minnesota Extension has a good rundown of common Minnesota weeds (including some guides to seedling ID).

Once your weeds are identified, dig into their biology -- this will help you figure out which cultural and physical management practices will be most effective. The SARE publication Manage weeds on your farm: a guide to ecological strategies has in-depth chapter on many common Minnesota weeds.

Cultural Weed Control

Rotation is important to keep soil-borne diseases like phytophthora and fusarium away, but can also be important as it allows for other crops that break weed cycles or have alternative herbicide chemistries available to be part of long-term weed management. For pumpkins and winter squash, a three year rotation is a good starting point.

Be careful when rotating with field crops, as many corn herbicides have a 18 or 24 month replant interval for specialty crops. If you swap land with a neighbor, it is worth some research and a conversation to figure out how their weed control can work for both of you.

For agritourism farms, parking, attractions, and customer access to fields can make rotation tricky. The increased popularity of sunflowers in recent years allows an opportunity to diversify crop rotation, weed control, and income timing.

Mechanical Weed Control

Tillage, especially before planting, is important for starting weed free. Both organic and conventional growers should consider preparing the field 2-4 weeks in advance, letting a round of weeds flush, then killing these weeds with very shallow tillage, flaming, or a burndown herbicide. This is called the stale seedbed method.

Another important time for mechanical weed control is before weeds set seed. Many weeds produce thousands of seeds that can survive in the soil for long periods of time (some species over a decade), depending on the species and soil conditions. For example, the average common lambsquarters plant sets over 70,000 seeds, eastern black nightshade produces 10,000, and marestail over 200,000. Hand-removing weed escapes before they set seed can prevent years of future weed problems.

Pre-Emergence Herbicide Options

For conventional growers, pre-emergence herbicides are a cornerstone of good weed control. These chemistries create a chemical barrier in the soil that recently germinated seeds cannot break through. This means young pumpkin plants start the season with minimal competition from weeds, allowing them to become well-established and get in a few weeks of solid growth. Once the pre-emergent herbicide is no longer active, the larger plants will be better able to out-compete mid- and late-season weeds. Labor for hand-weeding is expensive and hard to come by, and post-emergence chemistries have limitations and risks to the main crop, making pre-emergence herbicides worth considering.

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